Printer calibration has become an important topic as print resolution and reproduction accuracy improve in printers.
Calibration is used to compensate for reproduction inconsistencies that may arise due to manufacturing variations in ink drop weight, ink chemistry and the environmental effects of temperature and humidity on the printing process. For example, ink drop weight produced by different print heads often varies as a result of minute manufacturing differences in the size of the nozzles used in an inkjet print head, different resistor characteristics in the heater element used to eject the ink droplets in the inkjet print head, variations in the orifice shape, or any other difference from one print head to another. Differences in the ink chemistry combined with temperature and humidity also affect the final color appearance when applied to a print medium. To accommodate these factors, it is known to employ color calibration which modifies the number of ink drops during printing to make color printing more consistent.
Reproduction inconsistencies are not, however, simply limited to color. Non-uniformity in printing can also be caused by factors such as aerodynamic variations, temperature fluctuations within print dies and minor misalignment between adjacent print dies. This non-uniformity is especially evident in printers that have more than two dies, and increases with the number of print dies used by a printer. As illustrated in FIG. 1, such misalignment can also be visible as a light area (10) corresponding to a die-to-die boundary and is known as Light Area Banding (LAB).
Known methods and systems for compensating such non-uniformities have typically focused on directly addressing its causes. For example, it is Known to use different masking on die boundaries in an attempt to reduce aerodynamic fluctuations. Currently, no proposals have been made that seek to address the problem of temperature fluctuations.
Some known printers employ a pen alignment procedure which is supposed to correct misalignment between dies. However, due to noise in the system a non-zero value of misalignment still remains after such a pen alignment procedure. This value of misalignment may be larger when a known alignment procedure known as Automatic Pen Alignment (APA) is used to align dies in particular printers. Further, the model used in APA procedure is not perfect and there exist other effects such as media curl and ambient light that add to and alter the noise in the system.
It is therefore desirable to develop a calibration technique to cater for printing non-uniformity at die-to-die boundaries of a print head and to improve printing quality and/or accuracy.
Previously, different print masks have been used to hide the misalignment artifacts. Although using different masks can be useful in making misalignment less apparent, it cannot completely eliminate misalignment artifacts.